Diffusive transport in porous

Saltzman, W. M., Pasternak, S. H., and Langer, R., Micro-structural models for diffusive transport in porous polymers, in Controlled-Release Technology, ACS Symposium Series 348... 

Microstructural Models for Diffusive Transport in Porous Polymers... 

The results from this analysis can now be used to construct geometrically accurate models of the diffusive transport in porous polymers. Previous models of diffusion in these polymers have used an empirically determined tortuosity factor as a lumped parameter to account for the retardation of release by all mechanisms (7-8). 

At steady state, the rate and current density of an electrocatalyst in a MER are uniform. In a CER, however, reactant concentration declines along the reactor and current decreases under potentiostatic control for non-zero-order, single, or multiple reactions. Current nonuniformity in a CER becomes more pronounced with decreasing reduction or increasing oxidation potentials (60-62). With slow diffusive transport in porous catalysts, significantly lower potentials are necessary to reach the same degree of nonuniformity as in the absence of pore diffusion (61). 

The porosity or pore water volume fraction of total bed volume e (m m ) is obviously a key independent variable for assessing diffusive transport in porous media. The water that is contained in the bed is called the porewater or interstitial water because it fills the pores or interparticle spaces. It is the key phase for describing chemical mass transport interactions with the overlying water. Hence, all in-bed fluxes of dissolved constituents are transported in this fluid phase. 

The success of transport models must be measured by their ability to describe the results of flow and diffusion measurements in porous media. 

Vol. 1 Polymer Engineering Vol. 2 Filtration Post-Treatment Processes Vol. 3 Multicomponent Diffusion Vol. 4 Transport in Porous Catalysts... 

A number of different approaches have been taken to describing transport in porous media. The objective here is not to review all approaches, but to present a framework for comparison of various approaches in order to highlight those of particular interest for analysis of diffusion and electrophoresis in gels and other nanoporous materials. General reviews on the fundamental aspects of experiments and theory of diffusion in porous media are given... 

The objective of most of the theories of transport in porous media is to derive analytical or numerical functions for the effective diffusion coefficient to use in the preceed-ing averaged species continuity equations based on the structure of the media and, more recently, the structure of the solute. 

Kim, JH Ochoa, JA Whitaker, S, Diffusion in Anisotropic Porous Media, Transport in Porous Media 2, 327, 1987. 

Saez, AE Perfetti, JC Rusinek, I, Prediction of Effective Diffusivities in Porous Media Using Spatially Periodic Models, Transport in Porous Media 6, 143, 1991. 

Trinh, SH Arce, P Locke, BR, Effective Diffusivities of Point-Like Molecules in Isohopic Porous Media by Monte Carlo Simulation, Transport in Porous Media 38, 241, 2000. 

Surface diffusion is yet another mechanism that is often invoked to explain mass transport in porous catalysts. An adsorbed species may be transported either by desorption into the gas phase or by migration to an adjacent site on the surface. It is this latter phenomenon that is referred to as surface diffusion. This phenomenon is poorly understood and the rate of mass... 

Gas-diffusion electrode metal-air cells gas-transport in porous media carbon-based catalysts. 

In dense membranes, no pore space is available for diffusion. Transport in these membranes is achieved by the solution diffusion mechanism. Gases are to a certain extent soluble in the membrane matrix and dissolve. Due to a concentration gradient the dissolved species diffuses through the matrix. Due to differences in solubility and diffusivity of gases in the membrane, separation occurs. The selectivities of these separations can be very high, but the permeability is typically quite low, in comparison to that in porous membranes, primarily due to the low values of diffusion coefficients in the solid membrane phase. 

The species diffusivity, varies in different subregions of a PEFC depending on the specific physical phase of component k. In flow channels and porous electrodes, species k exists in the gaseous phase and thus the diffusion coefficient corresponds with that in gas, whereas species k is dissolved in the membrane phase within the catalyst layers and the membrane and thus assumes the value corresponding to dissolved species, usually a few orders of magnitude lower than that in gas. The diffusive transport in gas can be described by molecular diffusion and Knudsen diffusion. The latter mechanism occurs when the pore size becomes comparable to the mean free path of gas, so that molecule-to-wall collision takes place instead of molecule-to-molecule collision in ordinary diffusion. The Knudsen diffusion coefficient can be computed according to the kinetic theory of gases as follows... 

Compared to rivers and lakes, transport in porous media is generally slow, three-dimensional, and spatially variable due to heterogeneities in the medium. The velocity of transport differs by orders of magnitude among the phases of air, water, colloids, and solids. Due to the small size of the pores, transport is seldom turbulent. Molecular diffusion and dispersion along the flow are the main producers of randomness in the mass flux of chemical compounds. 

W. Jost, Diffusion in Solids, Liquids, Gases, p. 425, Academy Press, New York, 1960 R. Jackson, Transport in Porous Catalysts, p. 9, Elsevier, Amsterdam, 1977. 

Diffusion media (DM) are prone to flooding with liquid water. Although the DM is an essential component of PEFCs that enable distribution of species and collection of current and heat, little was known about capillary transport in DMs until recently. In Chapters 7 Gostick et al. provide a description of liquid water transport in porous DM due to capillarity and describe experimental techniques used to characterize DM properties. 

See, for example, M. L. Brusseau and P.S.C. Rao. Sorption nonideality during organic contaminant transport in porous media, Crit. Rev. Environ. Control 19 33 (1989) and W. W. Wood, T. F. Kraemer, and P. P. Hearn, Jr., Intragranular diffusion An important mechanism influencing solute transport in clastic aquifers Science 247 1569... 

Diffusion is the process of molecular transport associated with the stochastic movement of the individual diffusants. Diffusion coefficients in porous media may be defined on the basis of the generalized Fick s first law... 

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